A pulsed laser video imaging system and method is set forth in U.S. Pat. Nos. 4,720,747 and 4,851,918, issued on Jan. 19, 1988, and Jul. 25, 1989, respectively, both hereby incorporated by reference. These patents describe a video imaging system responsive to input signals representing a video image and employ one or more pulsed lasers, such as metal vapor lasers, to provide one or more monochromatic light sources.
The invention relates to a laser video display system and method. In particular, the invention relates to a full color, laser video image system wherein the pulsed laser light source comprises a green (G) laser with controlled multimode operation to minimize speckle, a tunable blue (B) laser and a red (R) laser with sum of frequency mix.
It is desirable to provide a new and improved pulsed laser projection system and method with one or more of the following improvements. The list of improvements include:
1. all solid state green laser design with controlled multimode operation to minimize speckle;
2. all solid state tunable blue laser design;
3. all solid state red laser design with sum of frequency mix;
4. diode laser pumped, all solid state, monochromatic red, green and blue light source with color space conversion;
5. uniform intensity line generation by a cylindrical asphere lens set;
6. two channel image output with achromatic image inversion;
7. remote image delivery by coherent fiber ribbon;
8. horizontal video line time adjustment (compression and expansion);
9. nonlinear control of vertical scanner (i.e., tangential correction); and
10. horizontal pixel time correction with sub-clock timing.
The various components of the improved laser video image system and method, and the differences and improvements from prior art systems and methods are set forth in detail below.
(1) All Solid State Green Laser Design with Controlled Multimode Operation to Minimize Speckle
The original invention described in U.S. Pat. No. 4,720,747 specifies the use of metal vapor lasers as light sources. U.S. Pat. No. 3,818,129, issued Jun. 18, 1974, to Yamamoto, incorporates use of a cw lamp pumped, repetitively Q-switched, frequency doubled Neodymium:Yttrium Alminum Garnet (Nd:YAG) laser (wavelength=532 nm) to be a light source for green, and it cites the short pulse duration and high average power as the primary reasons for the choice of the above-mentioned laser. However, U.S. Pat. No. 3,818,129 fails to mention another important factor affecting image quality, which is output beam quality from the above-mentioned laser.
Generally speaking, the output beam quality from the cw lamp pumped, repetitively Q-switched, frequency doubled, Nd:YAG laser with high output power tends to have a high multimode structure in transverse direction, which results in high beam divergence; therefore producing undesirable image blur at a screen. The above-mentioned laser can be constructed to produce near diffraction limited, single mode output (TEMoo mode) which minimizes the beam divergence; however, TEMoo mode output does maximize laser speckle effect, which is not desirable.
Then what is needed is the cw lamp pumped, repetitively Q-switched, frequency doubled Nd:YAG laser cavity design which produces controlled multimode output in transverse direction that minimizes the laser speckle and still produces reasonably crisp images on the screen surface because of optically manageable beam divergence. M2 is a measurable quantity which characterizes output beam spot size at far field and its divergence. When M2 is equal to one, the output beam is called diffraction limited beam or TEMoo mode, whereas when M2 is large (i.e., xcx9c100), then the output beam is said to have a high multimode structure. The acceptable range of M2 for the laser video display discussed in this invention is semi-empirically determined to be between 10 and 20. To achieve the acceptable range of M2=10-20, the cw lamp pumped, repetitively Q-switched, frequency doubled Nd:YAG laser has an intra-cavity aperture to strip excess modes, and the frequency doubling process is achieved by a Type II LBO (lithium triborate) or KTP (potassium titanyl phosphate) placed within the laser cavity. The schematic drawing of this green laser cavity is shown in FIG. 3.
(2) All Solid State Tunable Blue Laser
Blue light is produced by a frequency doubled Ti:Sapphire (Ti:Al2O3) laser, which is longitudinally pumped by the cw lamp pumped, repetitively Q-switched, frequency doubled Nd:YAG laser (wavelength=532 nm). The Ti:Al2O3 laser has broad range of near infrared emission; thus, it can be tuned to a specific wavelength by a set of birefringent plates, and for this particular application, the IR emission is tuned at 900 nm. The frequency doubling process is achieved by placing a Type I LBO or BBO (beta-barium borate) within the Ti:Al2O3 laser cavity (i.e., intra-cavity frequency doubling), which results in emission of blue light at 450 nm. Finally, the range of blue emission from this frequency doubled Ti:Al2O3 laser can be tunable by adjusting the angle of the birefringent plates. The schematic drawing of the frequency doubled Ti:Al2O3 laser is shown in FIG. 4.
(3) All Solid State Red Laser with Sum of Frequency Mix
A cw lamp pumped, repetitively Q-switched Nd:YAG laser produces primary laser radiation of 1064 nm. This 1064 nm radiation is used to pump the potassium titanyl arsenate (KTA) based intracavity optical parametric oscillation (OPO) and sum of frequency mix (SFM) mechanism to produce red light in wavelength between 626 nm and 629 nm. When KTA crystal is pumped by 1064 nm, it has been demonstrated to produce the signal (1520 nmxcx9c1540 nm) and the idler (xcx9c3540 nm) waves, and unlike KTP, the KTA does not exhibit reabsorption of the idler wavelength (xcx9c3540 nm); thus, relatively high conversion efficiency is expected from KTA based OPO once the pump beam exceeds OPO threshold. A separate Type III KTP will be used to achieve the sum of frequency mix process, and it has a phase match angle of 77xc2x0 for SFM process between the wavelength of 1520 nmxcx9c1540 nm and 1064 nm, producing the resultant red wavelength of between 626 nm and 629 nm. Similarly, Type II KTA or Type I LBO can be used to achieve sum of frequency mix (SFM) between 1520xcx9c1540 nm and 1064 nm to produce the desired red wavelength of 626xcx9c629 nm, instead of Type III KTP discussed above.
The lasing mechanism to generate 1064 nm radiation, and KTA based OPO, and subsequent KTP based SFM process can be placed in the same cavity structure (intra-cavity design) or the two can be separated, depending on peak power of 1064 nm radiation. The schematic drawing of the former cavity design is shown in FIG. 5.
(4) Diode Laser Pumped, All Solid State, Monochromatic Red, Green and Blue Light Source with Color Space Conversion
The all solid state red, green and blue laser designs discussed previously are based on cw lamp pump mechanism to produce primary laser radiation of 1064 nm from Nd:YAG crystal. However, diode laser pumped, all solid state red, green and blue laser light source described in U.S. Provisional Patent Application No. 60/032,269, filed Nov. 29, 1996 (Title: xe2x80x9cMonochromatic R,G,B Laser Light Source and Display Systems by Masayuki Karakawa), will also be used as an alternative light source to produce three primary colors. This diode laser pumped, all solid state red (wavelength: 626xcx9c629 nm), green (wavelength: 532 or 523.5 nm) and blue (wavelength: 450 or 447 nm) laser light source also incorporates digital color space conversion electronics circuit and produces a very short pulse at a high repetition rate.
(5) Uniform Intensity Line Generation by a Cylindrical Asphere Lens Set
In the laser video projection system described in U.S. Pat. Nos. 4,720,747 and 4,851,918, each pulse of laser light having Gaussian intensity distribution, from one or more laser source must be converted to a line by an optical set up, and enters into an acousto-optic cell which acts as a spatial light modulator. In this process it is important to have the optical set up, which converts the laser beam having circular cross section and Gaussian intensity distribution into a well-collimated, uniform intensity, thin line, in order to have good image quality at the screen and achieve maximum coupling with the acousto-optic cells.
In U.S. Pat. Nos. 4,720,747 and 4,851,918, the optical set up, which consists of a pair of cross-cylinder lenses with a collimator is suggested, and in U.S. Pat. No. 3,818,129, no particular optical set up is mentioned in this area. A pair of cross-cylinder lenses with a collimator approach does produce a line; however, the intensity distribution and the line thickness are not uniform across the line.
The improved optical set up disclosed herein consists of a cylindrical asphere lens and a pair of cylinder lenses cemented together to form a collimator set. The primary function of the cylindrical asphere lens is to convert a laser beam with circular cross section and Gaussian intensity distribution into a uniform intensity thin line, and the collimator set, made of a pair of cylinder lenses cemented back to back, keeps the line stray collimated at far field. An aspherical surface of the cylindrical asphere lens is given by equation:
X=cy2/[1+(1xe2x88x92c2y2(1+k))xc2xd]+A4y4+A6y6+A8y8+A10y10+ - - - 
where k is conic constant and c is curvature.
It is very important to note that inclusion of the higher order terms (i.e., A4y4, A6y6, A8y8, A10y10, - - -) are essential to produce uniform intensity distribution across the entire line; thus, they should not be overlooked. The higher order coefficients such as A4, A6, A8, A10, - - - are determined based on the precise spatial intensity distribution (a measurable quantity) of the source laser beam.
(6) Two Channel Image Output with Achromatic Image Inversion
For a certain application, it is desirable to project two identical images on two different screen surfaces by one projector. To achieve this effect, it is necessary to:
1. split the image in half by trichroic beam splitter, which transmits 50% of the image through and also reflects the remaining 50% of the image;
2. apply necessary achromatic corrections to the reflected image to compensate color differences among red, green and blue, since the coating on the trichroic beam splitter may not divide red, green and blue images exactly in half;
3. then invert the reflected image, so that its geometry is the same as the transmitted one; and
4. finally, two images, the transmitted and reflected ones, are vertically scanned by two galvanometer driven scanners, controlled by the single electronic circuit, so that the two images are always at sync.
The schematic drawing of optical set up to achieve two image output, with achromatic image inversion, as described above is shown in FIG. 6.
(7) Remote Image Delivery by Coherent Fiber Ribbon
For a certain applications, it is desirable to decouple a main portion of the laser video projector mechanism, which includes red, green and blue lasers, three acousto-optic cells, and other optical components up to zero order focus (ZOF) beam block and a projector head, which includes the galvanometer driven vertical scanner and projection optics, and then connect them with fiber cable.
Since prior to launching into fiber all the imaging functions, including line-thickness control, and R,G,B field matching have been performed, the fiber cable used here should be 1xc3x97N coherent fiber ribbon, where N should be a sufficiently large number (integer) and preferably close to the number of horizontal pixels. The coherent fiber ribbon set up is necessary to preserve already encoded video images from one end of the fiber to the other end.
Two different applications are envisioned for this remote image delivery method by 1xc3x97N coherent fiber ribbon. The first application is single projector/single display use, and the second application is single projector/multiple display use. The only difference between the two applications is that in the second application, the image is divided in half by a fused fiber image divider prior to launching into two separate 1xc3x97N coherent fiber ribbons, which are then connected to two separate projector heads.
The single projector/single display application is shown in FIG. 7 and the single projector/multiple display application is shown in FIG. 8.
(8) Horizontal Video Line Time Adjustment (Compression and Expansion)
In U.S. Pat. Nos. 4,720,747 and 4,851,918, electronic video line time compression (or data compression) concept is included. This allows the use of a smaller size aperture, acousto-optic cell; thus, enabling the input laser light to attain high diffraction efficiency into the first order when the input signal considered is NTSC video signal only.
However, there are many other video signal formats available now, and for certain input signals applications, it is desirable and necessary to expand the input video line time. For example, consider the input signal having the resolution of 1280 (H) pixels by 1024 (V) lines at 72 Hz refresh rate with progressive scan format (non-interlace format): The horizontal video line time for this input signal is approximately equal to 11-12 xcexcs; whereas, the necessary aperture time for the acousto-optic cell is approximately equal to 22 xcexcs, if the multi-line writing scheme is implemented. Furthermore, for an anisotropic, acousto-optic cell, such as TeO2 operating in sheer mode, there is not much difference in diffraction efficiencies into the first order between 11 xcexcs (horizontal lengthxcx9c6.8 mm) aperture and 22 xcexcs (horizontal lengthxcx9c13.6 mm). Therefore, what is needed is not just video line time compression, but video line time adjustment, which includes compression and expansion means to manipulate input video line time best suited for different applications.
The input video line time expansion can be achieved in a similar manner as video line time compression by:
1. storing input each horizontal video line data into line or frame buffers momentarily;
2. retrieving the stored data from the line or frame buffers with different rates from the input video signal;
3. sending the output video signal to one or more transducers attached to acousto-optic cells; and
4. when the output video signal rate is shorter than the input one, the signal is compressed; whereas, if the output video signal rate is longer than the input one, the signal is expanded in time domain.
(9) Nonlinear Control of Vertical Scanner
Generally speaking, a galvanometer based vertical scanner, driven by linear ramp signal, makes incremental vertical sweep, with equally spaced angular displacement for given oscillation rate, and its linearity (i.e., each angular displacement) is known to be excellent.
However, when the galvanometer based vertical scanner is used to project each horizontal video line onto the screen surface with relatively short throw distance, the equally spaced angular displacement motion of the vertical scanner, driven by a series of linear ramp signals, does not translate into equal spatial spacing of each horizontal line at the screen surface. The actual result is that the horizontal line spacing is closer together at the top and bottom of the screen, and they are more apart at the center of the screen. In order to achieve equal spatial spacing of each horizontal line at the screen surface, the galvanometer driven vertical scanner has to be driven by nonlinear ramp signals, providing unequally spaced angular displacement motion.
Refer to FIG. 9, which shows each angular displacement of the vertical scanner and the screen surface, the ith incremental angular displacement made by the vertical scanner resulting in the projection of the ith horizontal line onto the screen is given by the equation:
Ai=ArcTan [Tan (Aixe2x88x921+Aixe2x88x922+Aixe2x88x923+ - - - +A2+A1)+Tan A1]xe2x88x92(Aixe2x88x921+Aixe2x88x922+Aixe2x88x923+ - - - +A2+A1)
Where:
A1=ArcTan (L/D)
L=Identical spacing for each horizontal line at the screen surface=H/N
H=Height of the screen
N=Total number of horizontal lines=vertical resolution of the image
D=Distance between the scanner and the center of the screen
The nonlinear angular displacement of the vertical scanner given by the equation above is used to calculate each angular displacement corresponding to each projected horizontal line, and the results can be programmed into look up tables within firmware, then the programmed data can be used to provide necessary electronics correction to ramp signal (tangential correction) which drives the vertical scanner.
(10) Horizontal Pixel Time Correction with Sub-clock Timing
When the laser video projector described in this invention is used with relatively short throw distance from the screen surface, the projected image suffers from a distortion known as xe2x80x9cpin cushionxe2x80x9d effect.
This distortion originates from the fact that it takes a longer time for a pixel to reach the upper or lower section of the screen than to reach the center section of the screen; therefore, the pixels at the top and bottom section of the screen expand more than those at the center section, resulting in the image having bow-shaped left and right edges.
In order to correct this image distortion, pixel by pixel-based time correction is needed such a manner, that additional time is assigned to a group of pixels corresponding to the center section of the image; whereas, lesser time is assigned to a group of pixels corresponding to the four upper and lower corners of the image. This pixel time correction is implemented within the acousto-optic cell in the following manner:
Consider a case with a video pixel time equal to 28 ns. Normally, this pixel signal is generated by 36 MHz master clock (f=1/28 nsxcx9c36 MHz) and pixel time interval does not change across the given video line within the acousto-optic cell. However, the additional sub-clock circuit oscillating at 500 MHz will provide +/xe2x88x922 ns change into the master clock circuit oscillating at 36 MHz, and the result enables the combined clock signal circuits to generate pixel time varying (28+/xe2x88x922.N)ns (where N is integer=0, 1, 2, . . . ). With this additional sub-clock circuit, the pixel time can be adjusted across the given video line so that shorter pixel time is assigned to a group of pixels at the both edges and longer time is assigned to a group of pixels at the center section of the video line.
This pixel time correction with sub-clock circuit produces the predistorted image within the acousto-optic cell, which will compensate for the xe2x80x9cpin cushionxe2x80x9d problem at the screen surface, resulting in an image with straight line left and right edges as desired.
The invention will be described for the purpose of illustration only in connection with certain illustrated embodiments; however, it is recognized that various changes, modifications, additions and improvements may be made in the illustrative embodiments without departing from the spirit and scope of the invention.